This invention relates to improvements in the production of man-made staple fibre. Staple fibre can be produced by extruding a solution of a polymer through a spinneret to form a tow of continuous filaments and continuously cutting the filaments.
U.S. Pat. No. 5,582,786, for example, describes the production of a water-absorbent water-insoluble fibre by extruding an aqueous solution of a water-soluble polymer into a gaseous medium through a spinneret to form a tow of continuous extruded filaments which is collected at a moisture content of 8 to 25% based on the dry weight of the fibre. The fibre is further dried at a temperature no greater than 100xc2x0 C. before crosslinking the fibre at a temperature in the range 125 to 250xc2x0 C. to a degree sufficient that the crosslinked fibre is water-insoluble. The continuous filaments are cut into staple fibre while the moisture content is 8 to 25%, because completely dry fibre is brittle and tends to form fly. Cutting of the water-soluble continuous filaments at a moisture content of 8 to 25%, however, causes smearing of polymer on the cutter so that the cutter frequently needs to be cleaned, interrupting production.
A method according to the present invention of producing staple fibre by extruding a solution of a polymer through a spinneret to form continuous filaments, gathering the continuous filaments to form a tow and continuously cutting the filaments in a cutter to form staple fibre is characterised in that at least one jet of a gas is blown at the tow of continuous filaments before it enters the cutter.
While the method of the invention is particularly suitable for producing water-absorbent staple fibre of the type described in U.S. Pat. No. 5,582,786, it can be used for cutting any tow of man-made continuous filaments into staple fibre. The polymer solution which is extruded can for example be a solution of a synthetic polymer or a natural polymer. It can be dry spun, i.e. extruded into a gaseous medium, or wet spun, i.e. extruded into a regenerating bath. The method of the invention has particular advantages when applied to filaments spun (extruded) from acueous solution and/or still wet with aqueous solution, but it can also be applied to filaments spun from organic solvent solution. The tow of continuous filaments can be treated in tow form before cutting; for example a tow of continuous cellulose filaments can be carboxymethylated as described in WO-A-93/12275. The process of the invention is particularly suitable for cutting the resulting water-absorbent filaments into staple fibre.
The method of the invention is generally advantageous for cutting water-soluble or water-absorbent filaments. The process of the invention is also particularly suitable for cutting any tow of filaments which is cut in an uncured form, that is where the staple fibre is subsequently cured to harden the fibre, for example heated to crosslink the polymer of which the fibre is made, eg to form water-insoluble but water-absorbent fibre from a polymer which is water-soluble when extruded to form continuous filaments and cut. The method of the invention can alternatively be applied to fibres which have been crosslinked or to thermoplastic fibres which do not need crosslinking, particularly water-absorbent fibres of these types. The size of the tow may for example be from 1000 to 20000 tex. The individual filaments of the tow can for example be from 1.5 to 50 decitex; the filaments of higher decitex within this range generally cause more problems at the cutter, particularly in the case of filaments in uncured form. The method of the invention can be used when cutting any staple length, for example 2 to 80 mm, although problems at the cutter are most frequent when cutting short staple lengths such as 2 to 25 mm, especially 2 to 6 mm.
The gas blown at the tow is preferably air, although an alternative gas, for example nitrogen, can be used. The temperature of the gas blown at the tow is preferably below 50xc2x0 C., for example xe2x88x925 to +20xc2x0 C. The velocity of the gas blown at the tow should generally be sufficient to open the tow, that is to say to separate the filaments of the tow. The gas can for example be at a pressure of 20 to 100 or 150 psi (140 to 700 or 1050 kPa), preferably 30 to 80 psi (200 to 550 kPa)
The gas is preferably blown at the tow in a direction substantially normal to the direction of travel of the tow, generally at an angle to the direction of travel of the tow within the range 75-105xc2x0, so that the gas has no substantial forwarding or retarding effect on the tow.
The air or other gas is preferably blown at the tow of continuous filaments so that it impinges on the tow from opposite sides of the tow. The tow is preferably spread widthways, for example by a spreader bar or roller, before the air or other gas is blown at the tow, or tow from several spinning ends can be fed side by side to form a wide flat tow. The air or other gas is preferably blown at the tow from a series or row of holes or slots which are spaced apart in a direction normal to the direction of the travel of the tow, for example about 5-15 mm apart. Most preferably, the holes, slots or series or rows of holes or slots are spaced apart in the direction of travel of the tow, so that air is blown from a 2-dimensional array of holes. The holes in successive rows may be staggered so that each filament of the tow comes close to passing over at least one hole. Most preferably the tow passes between two opposed manifolds each having such an array of holes. Each hole is for example 0.1 to 2 mm in diameter, preferably 0.5-1 mm. Alternatively, one or more slots, for example of the dimensions and type used in an air knife, can be used. Such slots are generally less than 1 mm wide The slots are preferably arranged with their lengthwise direction perpendicular to the direction of travel of the tow. In this way the slot or slots can extend across the whole width of the tow. Passing air under pressure through such holes or slots causes adiabatic cooling so that the air impinging on the tow is cooler than the air entering the manifold or air knife. One or more manifold of holes and one or more air knife slots can be used in sequence, in either order, to treat the tow.
The tow is preferably under low tension as it passes the jets of air or other gas, that is to say the rollers feeding the tow to the blower and the rollers receiving the tow from the blower operate at substantially the same speed. A high tension will tend to prevent the air jets opening the tow, while any significant overfeed could lead to looping or interlacing of the tow.
The gas blowing process can be carried out at any position between the tow forming and tow cutting operations. It is most preferably carried out just before the cutter so that the tow entering the cutter retains the reduced moisture content and reduced temperature imparted by the gas blast.
The cutter is preferably a rotary cutter, suitably with blades rotating about an axis in approximately the direction of travel of the tow within a housing which constrains the tow. One example is a Neumag NMC 450. An alternative is a Fleischner F 514.
The problems overcome by the method of the invention include filaments clumping together to form chunks of polymer between the cutter blades and smearing of polymer from the fibres on the cutter blade surface, causing inefficiency in cutting and eventual jamming of the cutter. We believe that one effect of the blast of gas is to remove surface moisture (both external and interstitial) from the filaments. The gas also has a cooling effect; for example a tow may be cooled from 60xc2x0 C. to 50xc2x0 C. by gas blown at 10xc2x0 C. even when the tow is travelling at 200-800 m/minute.